Workpieces which at high service temperatures (800.degree. C. and over) are subjected to considerable mechanical stress of a static as well as dynamic nature have normally been made of costly materials difficult to machine, such as tantalum, tungsten, molybdenum or of hard metals or hard substances, such as carbides, nitrides or the like.
If it is intended to combine in workpieces the properties of highly heat-resistant alloys, such as creep strength, with improved surface properties, such as resistance to friction and wear, resort is conventionally made to coating the pieces with hard materials by chemical gaseous phase separation, pack cementing, plasma spraying or similar methods. In the resulting product, the strength of the workpiece is characterized by the mechanical properties of the base alloy. Surface properties, such as resistance to wear or corrosion, are governed by the properties of the hard substance used in the coating.
In actual technical applications, however, various problems can arise. For example, as a result of the very unlike coefficients of thermal expansion between the base material and the coating material, lateral stresses may occur under alternating thermal loads in service at the steel/hard substance interface. These may induce cracks in the coating and cause it to separate. Under mechanical peaking loads acting at right angles to the surface, the ductile base material under the coating threatens to yield, causing the thin and brittle hard coating to collapse. It has therefore been necessary to produce staged joining systems between the hard surface layer and the base material via multiple coatings. This normally involves a large number of process operations, however, and makes production complex and costly.